With recent oil shortages and soaring energy prices, electrochemical devices have been accredited as a clean and alternative energy source for a wide range of applications. Electrochemical devices are energy conversion cells that are usually classified as either electrolyzer or fuel cell. Fuel cell is a device that converts the chemical energy into electrical energy. Electrolyzer uses electrical energy to decompose water into hydrogen and oxygen gases. Both cells comprise two electrically conductive electrodes separated by an ion exchange membrane. The efficiency of fuel cell as well as water electrolyzer depends on the health and integrity of core component “membrane.” In particular, the research on anion exchange membranes has been of increased attention. It is due to the fact that the alkaline system is able to improve the reaction kinetics, efficiency of oxygen reduction reaction and allows the use of low cost, earth abundant electro-catalyst unlike the precious metal catalysts, which are the major limitations for the widespread implementation of the proton exchange membrane.
Hitherto the most anion exchange membranes are typically composed of a polymer backbone onto which the fixed cationic sites are attached. With reference to Géraldine Merle, Matthias Wessling, Kitty Nijmeijer, Journal of Membrane Science (2011), 377, 1-35, the properties of the membrane are intimately related to the polymer matrix (mechanical, thermal stability, cross linking density) and the nature, concentration of the fixed cationic charges (ionic conductivity, charge density, ion exchange capacity, and transport number). Examples of such membranes are depicted in U.S. Pat. No. 8,436,057 B1 (2013), U.S. Pat. No. 8,492,049 B2 (2013), and U.S. Pat. No. 7,888,397 B1 (2011). Despite its early success, significant strides, and tremendous progress, several drawbacks exist in addition to the high cost. According to Christopher G. Arges, Vijay Ramani, Peter N. Pintauro, The Electrochemical Society Interface, Summer (2010), 31-35, successful commercial production of an alkaline ion exchange membrane includes a robust synthetic route with the proper selection and positioning of cationic ion-exchange groups on the polymer, controlling the membrane morphology to improve mechanical properties, scalability, use of low cost materials and processing methods. Thus synthesizing of cation functionalized hydrocarbon-based polymer backbone is a technical challenge. Usually swelling occurs after alkalization step. i.e., replacements of halide counter ions by hydroxyl groups. In order to attain good ionic conductivity, if the ionic site is increased by enhancing the amount of cationic ion-exchange groups, its mechanical stability will be lost and vice versa. Moreover the electron withdrawing characteristics of the polymer matrix (Toshikatsu Sata, Makie Tsujimoto, Takanori Yamaguchi, Koji Matsusaki, Journal of Membrane Science (1996), 112, 161-170) tend to weaken the stability of the bulky ions and hence reduce the durability. Generation of radical species (peroxy or hydroxyl radicals) in or near the electrode will attack the polymer and chemically degrade it. Further, degradation process can be quickened by electron-withdrawing molecules in the vicinity of fixed cationic site. In addition to that, any change in the water content will result in swelling or shrinkage of membranes, which can lead to the impairment of the membrane-catalyst interface or even membrane breakage (Christopher G. Arges, Vijay Ramani, Peter N. Pintauro, The Electrochemical Society Interface, Summer (2010), 31-35) thus, limiting the use of the membrane at elevated temperatures (above 90° C.). It even requires high humidification to avoid electro-osmotic drag which causes degradation of the membrane, consequently a tremendous decrease in conductivity. Apart from the above mentioned shortcomings, there is always a thirst for inexpensive precursor, nontoxic compounds, solvents and easy reaction steps in order to process the technology in a large scale with low cost and less maintenance.
Thus, commercialization of anion exchange membrane technology (water electrolyzer, fuel cell) is dropped owing to economic factors, material problems and certain inadequacies in the operation of electrochemical devices (Erich Gulzow, Journal of Power Sources (1996), 61, 99-104.). The more fundamental daunting challenges that significantly preclude the successful application are:
i) Scalability, low cost and maintenance
ii) Good stability in alkaline media
iii) High ionic conductivity
iv) Mechanical and dimensional stability